Catalyst support material having high oxygen storage capacity and method of preparation thereof

ABSTRACT

Mixed oxides of cerium oxide and zirconium oxides having a high oxygen storage and high oxygen release rate are disclosed. The mixed oxides are made of polycrystalline particles of cerium oxide and zirconium oxide having a controlled domain structure on the subcrystalline level wherein adjacent domains within a single crystallite have a different ratio of zirconium and cerium. The mixed oxides are prepared by a co-precipitation technique using mixed salt solutions of cerium and zirconium having a solid content of at least 23%, based on an oxide basis.

FIELD OF THE INVENTION

[0001] The present invention relates to novel compositions based onmixed oxides of cerium oxide and zirconium oxide having a high oxygenstorage capacity. This invention also relates to a novel process for thepreparation of the mixed oxide compositions, and the method of using themixed oxide compositions as catalysts and/or catalyst supports, inparticular, for the purification and/or conversion of exhaust gases frominternal combustion engines.

BACKGROUND OF THE INVENTION

[0002] Cerium oxide has been widely employed as a promoter of a catalystfor purifying exhaust gases emitted from an internal combustion enginebecause of its good oxygen storage ability. Typically, to improve theoxygen storage capacity (OSC), cerium oxide is usually employed as smallparticles having a relatively high specific surface area. Unfortunately,however, cerium oxide tends to sinter and lose surface area under hightemperature conditions thereby losing their effectiveness as an oxygenstorage component.

[0003] More recently, the need to thermally stabilize cerium oxide basedcatalysts against deactivation at higher temperatures has focusedattention on doping cerium oxide with a wide range of metals oxides. Tothis end, numerous prior art references have proposed incorporatingzirconium oxide or other oxides of rare-earth elements into cerium oxideto slow down the sintering process and provide high surface areamaterials. For example, Japanese Patent Application 55,15/1992,discloses a process for preparing mixed cerium oxide and zirconium oxidewherein a solution containing trivalent cerium salt and zirconium saltis co-precipitated with a base in the presence of hydrogen peroxide. Theprocess provides mixed oxides of cerium and zirconium having a highspecific surface area and an excellent heat resistance.

[0004] It has also been proposed that pure solid solutions of ceriumoxide and zirconium oxide having a high surface area are required to beeffective oxygen storage components in automotive catalytic converters.Various cerium oxide/zirconium oxide compositions having high surfacearea have been reported.

[0005] For example, U.S. Pat. No. 5,693,299 discloses ceriumoxide/zirconium oxide mixed oxide having thermal stability, very highspecific surface area of at least 80 m²/g. The mixed oxides are obtainedby thermohydrolysis and possess a pure monophasic CeO₂ cubic crystallinehabit wherein zirconium is incorporated into the crystalline habit ofthe cerium oxide.

[0006] U.S. Pat. No. 5,607,892 also discloses cerium/zirconium mixedoxide particles having high stable specific surface area. The mixedoxides are obtained by intimately admixing a zirconium sol with a ceriumsol, precipitating the mixture with a base to recover a precipitate, andthereafter calcining the recovered precipitate. An oxygen storagecapacity, measured on a product calcined at 1,000° C., of only 2.8 mlCO/g CeO₂ (62.5 micromole O₂ per gram of CeO₂) was reported.

[0007] In order to meet stringent future emission standards, it isnecessary that cerium oxide based catalysts exhibit high OSC even afterexposure to temperature in excess of 1,000° C. Since cerium basedcatalysts exposed to such high temperatures typically lose surface area,there is a need to develop cerium based materials which have a high OSCindependent of surface area.

[0008] Further due to recent advances in engine control technology,newer engines possess a even tighter air-fuel ratio control resulting inrapid changes in oxygen partial pressure at the location of thecatalyst. Catalyst useful in such engines are not only required topossess a higher oxygen storage capacity than prior known catalysts, butalso a high rate of oxygen release in order to respond to suchfluctuations in oxygen partial pressure. Consequently, there exists aneed in the automotive industry for catalyst/catalyst support materialswhich possess both a high oxygen storage capacity and an increased rateof oxygen release under high temperature conditions.

SUMMARY OF THE INVENTION

[0009] Novel compositions based on mixed oxides of cerium and zirconiumhaving an exceptionally high oxygen storage and release capacity havebeen developed. Mixed cerium oxide/zirconium oxide in accordance withthe invention possess a nominally cubic, polyphasic crystalline habitbased on a uniquely controlled domain crystalline substructure.Unexpectedly, mixed oxide compositions in accordance with the presentinvention possess a high oxygen storage capacity independent of surfacearea.

[0010] Mixed oxide compositions in accordance with the inventioncomprise polycrystalline particles based on cerium oxide and zirconiumoxide. Crystallites comprising the polycrystalline particles arecomposed of regions or “domains” at the subcrystalline level havingvarying atomic ratios of cerium and zirconium. In accordance with thepresent invention, it has been found that when adjacent domains within asingle crystallite sufficiently vary in their atomic ratios of ceriumand zirconium, a unique crystalline sub-structure will be present whichpromotes increased oxygen storage and oxygen release.

[0011] Without wishing to be bound to any particular theory, it istheorized that the compositional variation between adjacent domainscauses adjacent domains to possess different lattice parameters. Thisdifference in lattice parameters is believed to result in localizedstrain at the domain boundaries. It is hypothesized that such localizedstrain along the boundaries of adjacent domains provide a network ofinternal pathways throughout the crystallites. It is believed that thepresence of these pathways permits oxygen to be rapidly absorbed intoand released from the bulk crystalline lattice, thereby providingincreased oxygen storage and release capability independent of theexternal surface area of the particles.

[0012] Accordingly, a major advantage of the present invention is toprovide novel cerium oxide/zirconium oxide compositions having aspecified domain crystalline substructure which promotes an increasedoxygen storage capacity and rate of oxygen release when compared toprior cerium oxide/zirconium oxide compositions.

[0013] Another advantage of the present invention is to provide novelcerium oxide/zirconium oxide compositions which exhibits a high oxygenstorage capacity independent of surface area.

[0014] It is also an advantage of the present invention to providecerium oxide/zirconium oxide compositions having a high oxygen storagecapacity which compositions do not require a pure mono-phasic cubicsolid solution of either cerium oxide dissolved in zirconium oxide orzirconium oxide dissolved in cerium oxide as heretofore taught by theprior art.

[0015] It is yet another advantage of the present invention to providenovel mixed oxides of cerium and zirconium which are highly effective ascatalyst/catalyst support for the purification of exhaust gases.

[0016] Still another advantage of the present invention is to provide aprocess for the preparation and use of the novel cerium oxide/zirconiumoxide compositions. Other advantages and objects of the presentinvention will be understood from the detail of the description,examples and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a plot of the Normalized Scattering Intensity I(Q)between 0 and 12 versus Q ranging from 0.0 to 2.5 Å⁻¹, as determined bya small angle X-ray scattering (SAXS) technique, for ceriumoxide/zirconium oxide compositions prepared in accordance with Example 1(), Comparative Example 1 (▾) and Comparative Example 3 (

) which plot shows the position of the first diffraction peak at Qequals 2.06 Å⁻¹ from which the scattering intensity is normalized.

[0018]FIG. 2 is a plot of the Normalized Scattering Intensity I(Q)between 0 and 200 versus Q ranging from 0.0 to 0.15 Å⁻¹, as determinedby a small angle X-ray scattering (SAXS) technique, for ceriumoxide/zirconium oxide compositions prepared in accordance with Example 1(), Comparative Example 1 (▴) and Comparative Example 3 (

).

DETAILED DESCRIPTION OF THE INVENTION

[0019] The present invention will now be explained in detail hereinbelow.

[0020] The term “oxygen storage capacity” (OSC) is used herein toindicate the amount of oxygen stored in a sample as determined bymeasuring weight loss using conventional Thermal Gravimetric Analysis(TGA). The sample is held at 500° C. in flowing air at a rate of 120 ccper minute for 60 minutes to fully oxidize it. The air stream is thenimmediately replaced with a mixture of 10% H₂ in nitrogen at the sametemperature and flow rate and held isothermally for an additional 60minutes. Oxygen storage capacity is determined by measuring the weightloss going from oxidizing to reducing conditions. The unit used tocharacterize the OSC is micromole O₂ per gram of sample.

[0021] The term “oxygen release rate” is used herein to indicate thevelocity at which the oxygen leaves the Ce/Zr particles as measured byTGA. The sample is held at 500° C. in flowing air at a rate of 120 ccper minute for 60 minutes to fully oxidize it. The air stream is thenimmediately replaced with a mixture of 10% H₂ in nitrogen at the sametemperature and flow rate and held isothermally for an additional 60minutes. Oxygen release rate is calculated from the first derivative ofthe weight loss versus time curve and then is normalized by surface areaof the particles. The unit used to characterize the oxygen release rateis mg-O₂/m²-min.

[0022] The term “polycrystalline particle” is used herein to indicate aparticle made up of more than two crystallites as measured byconventional X-ray diffraction.

[0023] The term “crystallite” is used herein to indicate regions withina particle having the same crystallographic orientation and structure asdetermined by line broadening using conventional X-ray diffraction.

[0024] The term “domain” is used herein to indicate a region or volumewithin a single crystallite having a homogenous or a substantiallyhomogenous composition as determined by small angle X-ray scattering(SAXS). In accordance with the present invention, the Ce:Zr ratio of adomain is controlled to be different from adjacent domains comprisingthe crystallite.

[0025] The term “subcrystalline structure” is used herein to indicate aregion within a single crystallite which consists of two or moredomains.

[0026] The term “polyphasic” is used herein to indicate a materialcontaining more than one crystalline phase. This phase may consist ofmore than one crystalline structure, e.g. cubic and tetragonal, or thesame structure but a different lattice parameter.

[0027] The term “inhomogeneity” is used herein to indicate adjacentdomains within a single crystallite having a different Ce:Zr atomicratio.

[0028] The term “surface area” is used herein to indicate the surfacearea of a particle as measured by standard BET analysis.

[0029] The term “aging” is used herein to indicate heating a sample forthe purpose of accelerating changes in the properties of the sample.

[0030] The term “Normalized Scattering Intensity I(Q)” is used herein toindicate the scattering intensity as determined by a small angle X-rayscattering (SAXS) measurement divided by a constant such that theintegrated intensity under the first diffraction peak centered atapproximately Q=2.06 Å⁻¹ is equal to one.

[0031] Mixed oxide compositions in accordance with the present inventionhave a polyphasic crystalline structure and are made of polycrystallineparticles. Each particle has a cerium oxide component and a zirconiumoxide component and is comprised of a plurality of crystallites. Eachcrystallite in a particle is comprised of a subcrystalline structurewhich comprises a plurality of domains wherein the Ce:Zr atomic ratio ofadjacent domains is different and is characterized by a specified degreeof inhomogeneity in relation to each other, when measured by small angleX-ray scattering (SAXS). It will be understood that the SAXS measurementis performed on a multi-particle sample and the data collectedrepresents an average distribution of the level of inhomogeneity betweendomains at the subcrystalline level of the particles in the sample. Itis therefore inferred from the SAXS data that the individual particleswill on the average possess the above-described structure.

[0032] In accordance with the invention, domains have an average size ofabout 10 to about 50 Å, preferably about 10 to about 30 Å in freshlyprepared material. After aging at 1000° C. for 5 hours the domains willhave an average size of about 10 to 50 Å.

[0033] The domains are distributed within crystallites of cerium oxideand zirconium oxide having an average crystallite size of about 40 toabout 200 Å, preferably about 50 to 120, Å, as readily determined usingX-ray diffraction, using the peak at 28-30° 20 after calcination at 900°C. for 4 hours. The crystallites in turn make up the polycrystallineparticles having an average particle size ranging from about 0.1 toabout 50 μm, preferably about 0.5 to 20 μm.

[0034] In general, mixed oxide particles of the present inventioncomprises about 80 to 20 weight % CeO₂ and about 20 to 80 weight % ZrO₂preferably about 40 to 60% weight CeO₂ and about 60 to 40 weight % ofZrO₂. In a preferred embodiment, the mixed oxide composition includes 50weight % CeO₂ and 50 weight % of ZrO₂. Optionally, mixed oxide particlesof the invention may comprise up to about 10 weight %, preferably up toabout 8 weight %, most preferably from about 2 to about 7 weight %, ofan additional metal oxide other than cerium. Suitable additional metaloxides, include but are not limited to, oxides of rare earths metalsother than cerium, calcium oxide and mixtures thereof. Suitable rareearth metal oxides include, but are not limited to, oxides of lanthanum,praseodymium, neodymium, samarium, gadolinium and yttrium.

[0035] Typically, the mixed oxide compositions of the invention have aspecific surface area after calcination at 500° C. for 2 hours of atleast 30 m²/g, more preferably of at least 40 m²/g, and even morepreferably of at least 50 m²/g, and typically will range from about 30to about 120, preferably from about 40 to about 100, most preferablyfrom about 50 to 90, m²/g. After aging at 1000° C. at 4 hours, thespecific surface area is no more than 10 m²/g, preferably no more than 5m²/g, most preferably no more than 3 m²/g, and typically ranges fromabout 10 to about 1, preferably from about 5 to about 1, most preferablyfrom about 3 to about 1, m²/g.

[0036] Advantageously, mixed oxides of the present invention exhibitsimultaneously a high oxygen release rate and a high oxygen storagecapacity. Mixed oxides of the present invention have an oxygen storagecapacity measured isothermally at 500° C. of typically at least 260 μmolO₂/g sample, preferably, greater than 300 μmol O₂/g sample, even morepreferably, greater than 315 μmol O₂/g sample and most preferably,greater than 330 μmol O₂/g sample, after aging at 1000° C. for 4 hours.Typically, the mixed oxides of the invention will possess an OSC rangingfrom about 260 to about 800, preferably about 300 to about 600, mostpreferably about 350 to about 450, μmol O₂/g sample, after aging at1000° C. for 4 hours.

[0037] The mixed oxides of the present invention have a high oxygenrelease rate of typically greater than 1.0 mg-O₂/m²-min, preferablygreater than 2.0 mg-O₂/m²-min, most preferably greater than 5.0mg-O₂/m²-min, after aging at 1000° C. for 4 hours. Typically, the mixedoxides will possess an oxygen release rate ranging from about 1 to about100, preferably from about 2 to about 50, most preferably from about 5to about 10, mg-O₂/m²-min, after aging at 1000° C. for 4 hours.

[0038] The properties of increased oxygen storage and release exhibitedby the cerium oxide/zirconium oxide compositions of the invention isachieved by controlling the compositional variation in Ce:Zr atomicratios of adjacent domains within a single crystallite such thatdifferent lattice parameters are created between adjacent domains. Aswill be understood by those skilled in the art, some domains will becerium oxide rich, i.e., consist predominately of cerium oxide withzirconium oxide dissolved in cerium oxide, whereas other domains will bezirconium oxide rich, i.e., consist predominately of zirconium oxidehaving cerium oxide dissolved in the zirconium oxide. However, if thecompositions of adjacent domains is too homogenous or too inhomogenous,the required domain structure will not be present in the mixedcompositions to provide the desired oxygen storage and release ability.Thus, the degree of compositional variation or inhomogeneity betweenadjacent domains is important to achieve compositions having increasedoxygen storage capacity simultaneously with increased oxygen releaserates independent of surface area.

[0039] The degree of compositional variation between adjacent domainsmay be determined by using SAXS as described herein below.

[0040] While the goal of conventional X-ray scattering is to determinethe crystal structure and the positions of atoms, the purpose of theSAXS measurement is to probe local structural features at a scale largerthan the atomic distance, usually in tens and hundreds of angstroms. Thescattering angle 2θ between the incident beam and the detector isrelated to the scattering vector Q as Q=(4π/λ)sinθ, where λ is the X-raywavelength. The magnitude of the scattering vector Q defines thecharacteristic length which is probed by X-rays as π/Q. By measuringX-ray diffraction intensity at lower angles, thus at smaller scatteringvectors Q, one can probe spatially extended structural features inmaterials. To avoid the interference between the incident beam and theX-ray scattering intensity which is being measured at a very low angle,SAXS requires very stringent collimation conditions on the incidentbeam. Thus the SAXS measurement cannot be done with a standard X-raydiffractometer. For the measurement described in this patent, however,it is necessary at the same time to measure the scattering intensity atrelatively higher angles, in order to normalize the intensity as will beexplained in detail below. Synchrotron X-ray radiation provides an idealmeans to achieve this goal, since its low-divergence, high-intensityincident beam facilitates the collimation, allowing measuring the X-rayscattering intensity through a wide range of angles, and minimizesmeasurement time. The SAXS scattering measurements described herein wereperformed at the beamline X-7A of the National Synchrotron Light Sourceat the Brookhaven National Laboratory, Upton, N.Y. The incident X-rayswith the wavelength of λ=0.912 Å illuminated sample materials packedbetween the thin layers of capton, a polymer material highly transparentto X-rays. Typical thickness of the samples was in the range of 10 to100 μm. The scattering intensity was measured with a standard detectorfor the angles 0.66°<2θ<25.15° which correspond to Q in the range ofabout 0.08 Å⁻¹ to about 3.0 Å⁻¹. The incident X-ray beam was collimatedin such a way that the contribution of the incident beam to thescattered intensity measured even at the lowest angle of 2θ=0.66° wasnegligible. The X-ray scattered intensity was collected for severalseconds at each measurement point.

[0041] The intensity of the SAXS from the systems with phase separationobeys Porod's law,

I(Q)=K/Q ⁴  (1)

[0042] where I(Q) is the Normalized Scattering Intensity. In FIG. 1 theplot of the Normalized Scattering Intensity I(Q) versus Q shows a firstdiffraction peak centered at approximately Q=2.06 Å⁻¹ from which thescattering intensity was normalized.

[0043] In Equation (1) K is given by Equation (2):

K=2π(Δρ)² S  (2)

[0044] where Δρ is the difference in the electron density between thetwo phases, S is the interfacial area between the phases and is measuredin units of Å². At a given value of S the magnitude of I increases withincreasing Δρ, which in the case of (Ce, Zr)O₂ mixed oxides is primarilydue to compositional variation between domains within a crystal. If oneconsiders a two-phase sample with compositions (Ce_(1−X1)Zr_(X1))O₂ inone phase and (Ce_(1−X2)Zr_(X2))O₂ in the other, then Δρ is equal to0.49(X1−X2). The magnitude of the intensity at a specific Q is a measureof the compositional inhomogeneity.

[0045] The logarithmic plot of Equation (1) is a straight line describedby Equation (3):

ln(I(Q))=ln(K)−4 ln(Q)  (3)

[0046] Therefore, for the scattering intensities collected fromdifferent samples and normalized in the same way the slope of theresulting straight lines is −4, whereas the intercept depends on theparameters Δρ and S.

[0047] To determine the degree of inhomogeneity within thesubcrystalline domain structure of mixed oxides in accordance with thepresent invention, a SAXS measurement is carried out for scatteringvectors Q ranging from about 0.08 Å⁻¹ to about 3.0 Å⁻¹. The NormalizedScattering Intensity I(Q) is then plotted as a function of thescattering vector Q, as defined above, in the unit of Å⁻¹, rather thanas a function of 2θ, as is usually practiced.

[0048] Mixed oxides in accordance with present invention possess thecritical degree of inhomogeneity when they exhibit a NormalizedScattering Intensity I(Q) ranging from about 49 and about 119,preferably from about 50 to about 100, most preferably from about 54 toabout 85, when scattering Q is 0.10 Å⁻¹. Typically, the slope of thestraight line portion of the logarithm plot of the Normalized ScatteringIntensity, ln(I(Q)), as a function of logarithm of the scatteringvector, n(Q), at −2.5<ln(Q)<−1, is about −4.0, and therefore complieswith Porod's Law. It is well known by one skilled in the art that SAXSdata which conforms to Porod's Law will be most predictive of thedesired domain substructure.

[0049] In accordance with the present invention, the mixed oxides aretypically prepared by a co-precipitation of a mixed salt solutioncontaining a cerium salt and a zirconium salt dissolved in a suitablesolvent, e.g. water or an organic solvent. In a preferred embodiment,the solvent is water.

[0050] The solids concentration of mixed salt solutions used to preparethe mixed oxide of the present invention is important. If the solidscontent is too low, a subcrystalline structure having the desired degreeof inhomogeneity will not form. Thus, the solids content of the solutionis controlled to ensure the desired inhomogeneity. Typically, the mixedsalt solution will possess a solids concentration sufficient tofacilitate formation of the desired domain structure. Preferably, themixed salt solution will have a concentration of greater than about 23weight % solids, even more preferably, greater than about 25 weight %solids, and most preferably greater than about 27 weight % solids, basedon an oxide basis. Typically, the concentration of the mixed saltsolution ranges from about 24 weight % to about 39 weight % solids,preferably, about 25 weight % to about 29 weight % solids, based on anoxide basis.

[0051] Mixed salt solutions useful to prepare mixed oxides in accordancewith the present invention may be prepared by any conventional method.Typically, the mixed salt solution is prepared by mixing a cerium saltwith a zirconium salt in a manner and under conditions sufficient todissolve all or substantially all of the solids content in a suitablesolvent. In one embodiment, the mixed salt solution is prepared by amixing a cerium salt with an aqueous zirconium salt solution having acation to anion molar ratio of typically 1:1 to 1:2. For example, whenthe zirconium salt is zirconium oxynitrate, the zirconium salt solutionwill have a cation to anion molar ratio of typically, 1:2. On the otherhand, when the zirconium salt is zirconium hydroxy nitrate, thezirconium salt solution will have a cation to anion molar ratio oftypically 1:1.

[0052] In another embodiment of the invention, the mixed salt solutionis prepared by dissolving cerium carbonate into an aqueous zirconiumsalt solution to provide a solution having a 1:2 cation to anion molarratio and thereafter adding to the solution a minimum amount of an acidsufficient to dissolve all or substantially all of the carbonate asevidenced by a clear or transparent solution.

[0053] Suitable cerium and zirconium salts which may be employed toprepare mixed salt solutions useful in the process of the presentinvention include, but are not limited to, nitrates, chlorides,sulfates, carbonates and the like. Additional oxide components, i.e.dopants, may be added to the mixed salt solution in any soluble form.

[0054] Precipitation of the mixed salt solution may be accomplished bytreating the solution with a base, preferably ammonia, with agitation toprecipitate the corresponding hydroxide. The pH during precipitation isbasic, e.g. the pH typically ranges from about 8 to 11.

[0055] Following precipitation, the resulting precipitate is treatedwith an oxidizing agent in an amount sufficient to completely orsubstantially oxidize any Ce⁺³ to Ce⁺⁴. Suitable oxidizing agentsinclude but are not limited to, an aqueous solution of bromine, hydrogenperoxide, sodium bromate, sodium hypochlorite, ozone, chlorine dioxide,and the like. The preferred oxidizing agent is hydrogen peroxide.Typically, the precipitate is treated with an aqueous solution ofhydrogen peroxide in an amount sufficient to provide a molar ratio ofhydrogen peroxide to Ce of typically from about 0.25 to about 1.Preferably, the aqueous hydrogen peroxide solution is a dilute hydrogenperoxide having less than about 35 wt % of hydrogen peroxide. Typically,dilute hydrogen peroxide is added in an amount sufficient to provide amolar ratio of hydrogen peroxide to Ce of from about 0.5 to about 1.

[0056] It is desirable that the temperature during both theprecipitation and oxidization steps not exceed 80° C., preferably 70° C.and most preferably 60° C. In a preferred embodiment of the invention,the temperature during the precipitation or the oxidization step willtypically range from about 20° C. to about 70° C., and preferably fromabout 30° C. to about 60° C. Following precipitation, the precipitate isoptionally aged at a temperature of typically from about 70° C. to 100°C. for about thirty minutes to about 5 hours.

[0057] The resulting precipitate is filtered and then washed with waterto provide a filter cake. The filter cake is dried using anyconventional techniques to provide a free-flowing powder. In a preferredembodiment, the washed precipitate is re-slurried in water and theresulting slurry is subjected to spraying drying. The dried precipitateis thereafter calcined at a temperature of about 500° C. to about 600°C. for about 30 minutes to about 6 hours, preferably about 1 to about 4hours, most preferably about 2 to about 3 hours, to form a mixed oxidein accordance with the invention.

[0058] Optionally, a dopant may be added to the mixed oxide. When added,the dopant may be added at any point during preparation of the mixedoxides. Preferably, the dopant is added following precipitation butbefore or after calcination of the mixed oxide. Suitable dopants includeGroup VIII transition metals in the form of oxides, salts and the like.Preferably, the dopants include nickel, palladium or platinum, withpalladium and platinum being the most preferred. Typically, dopants areadded in amounts sufficient to provide from about 15 ppm to about 1000ppm, based on the weight of the mixed oxide, in the final mixed oxideproduct. It is desirable to add a dopant for the purposes offacilitating testing.

[0059] The calcined mixed oxide may thereafter be milled using an impactmilling technique to achieve the desired particle size. Suitable millingtechniques include, but are not limited to, high-energy ball milling,Spex milling, fluid energy milling and the like.

[0060] The increased oxygen storage capacity and oxygen release rate ofthe mixed oxide compositions of the present invention permit them to beused for numerous applications. In particular the mixed oxides of theinvention are well suited for catalysis applications, as catalyst and/orcatalyst supports. In a preferred embodiment mixed oxide compositionsaccording to the invention are used as constituents of a catalyst forthe treatment or conversion of exhaust gases emanating from internalcombustion engines. For this application, the mixed oxide compositionsof the invention are generally admixed with alumina before or afterimpregnation by catalytically active elements, such as noble metals.Such mixtures are then either shaped to form catalyst, for example inthe form of beads, or used to form a coating of a refractory body suchas a ceramic or metallic monolith, this coating per se being well knownin this art as a “washcoat”, as described for example in U.S. Pat. Nos.5,491,120; 5,015,617; 5,039,647; 5,045,521; 5,063,193; 5,128,306;5,139,992; and 4,965,245, said references being herein incorporated byreference.

[0061] To further illustrate the present invention and the advantagesthereof, the following specific examples are given. The examples aregiven as specific illustrations of the claim invention. It should beunderstood, however, that the invention is not limited to the specificdetails set forth in the examples.

[0062] All parts and percentages in the examples as well as theremainder of the specification are by weight unless otherwise specified.

[0063] Further, any range of numbers recited in the specification orclaims, such as that representing a particular set of properties, unitsof measure, conditions, physical states or percentages, is intended toliterally incorporate expressly herein by reference or otherwise, anynumber falling within such range, including any subset of numbers withinany range so recited.

EXAMPLES

[0064] The oxygen storage capacity (OSC) recited in the examples weredetermined by measuring weight loss using conventional ThermalGravimetric Analysis. The Samples are held at 500° C. in flowing air for60 minutes to fully oxidize it before switching to a mixture of 10% H₂in nitrogen and held isothermally for an additional 60 minutes. Oxygenstorage capacity is determined by weight loss going from oxidizing toreducing conditions.

[0065] All oxygen release rates cited in the examples were determined bycalculating the first derivative of the weight change versus timeprofile of the OSC measurement and normalizing it by the surface area ofthe sample.

EXAMPLE 1

[0066] 586 g of cerium (III) carbonate (49.5% oxide) was dissolved in1105 g of an aqueous (20%) zirconyl nitrate solution and 310 g ofconcentrated nitric acid. The final solution contained 26.7 wt % solidsas oxides. The solution was allowed to stir overnight to completelydissolve the carbonate. 93 g of this solution was poured into 400 ml of5N ammonia solution at a temperature of 40 C. under continuousagitation. The final pH after all of the nitrate solution was added wasapproximately 9. The slurry was mixed for 30 minutes at 40° C. afterwhich time, 52 g of a 3 wt % aqueous hydrogen peroxide solution wasadded. The molar ratio of hydrogen peroxide to cerium oxide was 0.25.

[0067] The precipitate was washed with 5 volume equivalents of hot DIwater. The ammonium nitrate was washed from the precipitate to aconductivity below 5 mS/cm.

[0068] The filter cake was diluted with water in a ratio of 1:1 to forma slurry and the aqueous slurry was spray dried to obtain a powder. Thedried powder was calcined at 500° C. for 1 hour to yield a final mixedoxide composition of 42 wt % zirconium oxide and 58 wt % cerium oxide.The powder was analyzed using small angle X-ray scattering (SAXS). Thepowder exhibited a Normalized Scattering Intensity I(Q) at Q=0.1 Å⁻¹ of57 as shown in FIG. 2. The Normalized Scattering Intensity I(Q) versus Qwas plotted in FIG. 1.

[0069] For the purpose of measuring the OSC, 15 ppm palladium wasimpregnated into the calcined mixed oxide powder as an aqueous nitratesolution and calcined at 500° C. The powder was aged at 1000° C. for 4hours and the OSC was measured using the previously described TGA test.The aged surface area was 1.0 m²/g, the oxygen storage capacity (OSC)was 363 μmole O₂/g sample and the oxygen release rate was 1.8mg-O₂/m²-min.

EXAMPLE 2

[0070] A filter cake was prepared using the procedure as described inExample 1 except when the filter cake was diluted with water in a ratioof 1:1 to form a slurry, the 15 ppm palladium was added to the slurry asan aqueous nitrate solution prior to spray drying. The dried powder wascalcined at 500° C. for 1 hour. The powder was aged at 1000° C. for 4hours and the OSC was measured using the previously described TGA test.The aged surface area was 1.0 m²/g, the oxygen storage capacity (OSC)was 376 μmole O₂/g sample and the oxygen release rate was 7.5mg-O₂/m²-min. The Normalized Scattering Intensity I(Q) was 57 at Q=0.1Å⁻¹.

EXAMPLE 3

[0071] 586 g of cerium (III) carbonate (49.5% solids) was dissolved in1105 g of an aqueous zirconyl nitrate solution (20 wt % solids) and 310g of concentrated nitric acid. The mixed oxide solution had aconcentration of 26.7% solids. 93 g of this solution was precipitated in400 ml 5N ammonia solution at 60C. After 30 min. of agitation, 1000 mlof a 3 wt % aqueous hydrogen peroxide solution were added to the slurry.The molar ratio of hydrogen peroxide to cerium oxide was 0.25.

[0072] The slurry was filtered and washed with 3 liters of DI water at70° C. The filter cake was re-slurried in water and 15 ppm of Pd, as thenitrate, was added to provide the final mixed oxide with 15 ppm Pd. Theresulting mixture was spray dried and calcined at 500° C. for 1 hour.The final composition contained 42 wt % zirconium oxide and 58 wt %cerium oxide.

[0073] The powder was aged at 1000° C. for 4 hrs. The 500° C. TGAreduction procedure gave an OSC of 342 μmole O₂/g sample and an oxygenrelease rate of 1.9 mg-O₂/m²-min. The Normalized Scattering IntensityI(Q)was 70 at Q=0.1 Å⁻¹.

EXAMPLE 4

[0074] A filter cake was prepared using the procedure in Example 3except that the aqueous filter cake slurry was doped with 100 ppm Pd, asthe nitrate, prior to spray drying. The spray-dried powder was calcinedand aged as in Example 3. The scattering intensity was 69 at Q=0.1 Å⁻¹,the OSC was 350 μmole O₂/g sample and the oxygen release rate was 50.5mg-O₂/m²-min.

EXAMPLE 5

[0075] A filter cake was prepared using the procedure in Example 3except that the aqueous filter cake slurry was doped with 1000 ppm Ni,as the nitrate, prior to spray drying. The spray-dried powder wascalcined and aged as in Example 3. The Normalized Scattering IntensityI(Q)was 69 at Q=0.1 Å⁻¹, the OSC was 308 μmole O₂/g sample and theoxygen release rate was 1.2 mg-O₂/m²-min.

EXAMPLE 6

[0076] A mixed nitrate solution was made by combining 930 g of anaqueous cerium (III) nitrate solution (28.3 wt % oxide) with 900 g of anaqueous zirconium hydroxy nitrate solution having a Zr:NO₃ ratio ofapproximately 1:1 (25.3 wt % oxide). The mixed solution had a solidscontent of 26.8 wt % on an oxide basis. The final oxide composition was52.5 wt % CeO₂ and 47.5 wt % ZrO₂.

[0077] The solution was added to 8 liters of 5N ammonia at 40° C. Theprecipitated hydroxide was treated with 1000 ml of a 3 wt % aqueoushydrogen peroxide solution.

[0078] The molar ratio of hydrogen peroxide to cerium oxide was 0.25.The filter cake was re-slurried in water and 15 ppm of Pd, as thenitrate, was added. The resulting mixture was spray dried and calcinedat 500° C. for 1 hour. The calcined powder was aged at 1000° C. for 4hours prior to measuring the available oxygen of the sample at 500° C.using the gravimetric apparatus. The OSC was 379 μmole O₂/g sample andthe oxygen release rate was 14.3 mg-O₂/m²-min. The Normalized ScatteringIntensity I(Q)was 55 at Q=0.1 Å⁻¹.

EXAMPLE 7

[0079] A mixed nitrate solution was made by mixing 633 g of cerium (III)carbonate (55 wt % oxide) was dissolved in 570 g of 70 wt % nitric acidand 142 g of DI water to make a cerium (III) nitrate solution having 29wt % solids. This was mixed with 965 g of an aqueous zirconium hydroxynitrate solution (26.1 wt % oxide) having a Zr:NO₃ ratio ofapproximately 1:1. The concentration of the mixed nitrate solution was27.7 wt % oxide solids. The final oxide composition was 58.9% CeO₂ and41.2% ZrO₂. The solution was added to 8 liters of 5N ammonia at 40° C.with agitation. The precipitated hydroxide was treated with 1000 ml of a3% aqueous hydrogen peroxide solution (H₂O₂/CeO₂=0.25M) prior tofiltration and washing of the ammonium nitrate. The filter cake wasreslurried with water and doped with 15 ppm of Pd as the nitrate. Theslurry was spray dried and calcined at 500° C. for 1 hour. After agingthe powder at 1000° C. for 4 hours, the OSC of the sample at 500° C.using the gravimetric apparatus was measured to be 377 μmole O₂/g sampleand the oxygen release rate was 7.5 mg-O₂/m²-min. The NormalizedScattering Intensity I(Q) was 77 at Q=0.1 Å⁻¹.

EXAMPLE 8

[0080] A mixed nitrate solution was made by mixing 67.1 g of an aqueouscerium (III) nitrate solution (28 wt % oxide) with 22.6 g of an aqueouszirconium hydroxy nitrate solution having a Zr:NO₃ ratio ofapproximately 1:1 (26.1 wt % oxide) yielded a final mixed nitratesolution concentration of 27.5 wt % solids on an oxide basis. The finaloxide composition was 70 wt % CeO₂ and 30 wt % ZrO₂. The solution wasadded to 400 ml of 5N ammonia at 40° C. and stirred for 30 min. Theprecipitated hydroxide was treated with a solution of 6.25 g of hydrogenperoxide in 45 g of DI (H₂O₂/CeO₂=0.25M) prior to filtration and washingof the ammonium nitrate. The filter cake was reslurried with water anddoped with 15 ppm of Pd as the nitrate. The slurry was spray dried andcalcined at 500° C. for 1 hour and aged at 1000° C. for 4 hours. The OSCof the sample at 500° C. using the gravimetric apparatus was 310 μmoleO₂/g sample. The oxygen release rate was 5.2 mg-O₂/m²-min. TheNormalized Scattering Intensity I(Q) was 70 at Q=0.1 Å⁻¹.

EXAMPLE 9

[0081] The nitrate solutions were prepared and precipitated the same asin Example 6. Lanthanum nitrate was added to the cerium and zirconiumnitrate to give a final solids content of the mixed nitrate solutionequal to 27.3 wt % oxide. The precipitation, drying and calcinationbeing carried out as in Example 6 to give a final oxide composition of51 wt % CeO₂, 44 wt % ZrO₂ and 5 wt % La₂O₃. After aging the powder at1000° C. for 4 hours the oxygen storage capacity based on the TGAmeasurement was 391 μmole O₂/g sample. The oxygen release rate was 3.4mg-O₂/m²-min. The Normalized Scattering Intensity I(Q) was 92 at Q=0.1Å⁻¹.

EXAMPLE 10

[0082] 46.5 g of an aqueous cerium (III) nitrate solution (28.5 wt %solids) was mixed with 61.8 g of an aqueous zirconium nitrate solution(20 wt % solids) to give a mixed solution concentration of 23.6 wt %solids on an oxide basis. The solution was poured into 400 ml of 5Nammonia solution at a temperature of 60° C. under continuous agitation.The slurry was mixed for 30 minutes at 60° C. after which time, 25 g ofa 30 wt % aqueous hydrogen peroxide solution was added.

[0083] The precipitate was washed with 3 liters of hot DI water. Thefilter cake was diluted with water in a ratio of 1:1 to form a slurryand 15 ppm palladium was added as a nitrate solution prior to spraydrying. The dried powder was calcined at 500° C. for 1 hour to yield afinal mixed oxide composition of 48.4 wt % zirconium oxide and 51.6 wt %cerium oxide.

[0084] The powder was aged at 1000° C. for 4 hrs to give a producthaving a surface area of <1 m²/g. The oxygen storage capacity (OSC) was339 μmole O₂/g sample and the oxygen release rate was 15.0 mg-O₂/m²-min.The sample was analyzed using small angle X-ray scattering (SAXS) asdescribed above and exhibited a Normalized Scattering Intensity I(Q) atQ=0.1 Å⁻¹ of 54.

EXAMPLE 11

[0085] A mixed nitrate solution was made by combining 17.2 g of anaqueous cerium (m) nitrate solution (29 wt % oxide) with 67 g of anaqueous zirconium hydroxy nitrate solution having a Zr:NO₃ ratio ofapproximately 1:1 (26.1 wt % oxide). To this solution was added 3.5 g of28.5 wt % lanthanum nitrate and 4.85 g of 31 wt % yttrium nitrate. Themixed solution had a solids content of 27 wt % on an oxide basis. Thefinal oxide composition was 20 wt % CeO₂, 70 wt % ZrO₂, 4 wt % La₂O₃ and6 wt % Y₂O₃. The solution was added to 300 ml of 5N ammonia at 40° C.The precipitated hydroxide was treated with 51 ml of a 3 wt % aqueoushydrogen peroxide solution.

[0086] The molar ratio of hydrogen peroxide to ceria was 0.25. Thefilter cake was re-slurried in water and 15 ppm of Pd, as the nitrate,was added. The resulting mixture was spray dried and calcined at 500° C.for 1 hour. The calcined powder was aged at 1000° C. for 4 hours priorto measuring the available oxygen of the sample at 500° C. using thegravimetric apparatus. The OSC was 260 μmole O₂/g sample and theNormalized Scattering Intensity I(Q) at Q=0.1 Å⁻¹ was 85. The oxygenrelease rate was 5.8 mg-O₂/m²-min.

EXAMPLE 12

[0087] A mixed nitrate solution was made by combining 88.9 g of anaqueous cerium (III) nitrate solution (28.3 wt % oxide) with 84.9 g ofan aqueous zirconium hydroxy nitrate solution having a Zr:NO₃ ratio ofapproximately 1:1 (26.1 wt % oxide). To this solution was added 6.25 gof praseodymium carbonate and 1.5 g of nitric acid. The mixed solutionhad a solids content of 26.5 wt % on an oxide basis. The final oxidecomposition was 50 wt % CeO₂ 44 wt % ZrO₂ and 6 wt % Pr₆O₁₁. Thesolution was added to 700 ml of 5N ammonia at 40° C. The precipitatedhydroxide was treated with 103 ml of a 3 wt % aqueous hydrogen peroxidesolution.

[0088] The molar ratio of hydrogen peroxide to ceria was 0.25. Thefilter cake was re-slurried in water and 15 ppm of Pd, as the nitrate,was added. The resulting mixture was spray dried and calcined at 500° C.for 1 hour. The calcined powder was aged at 1000° C. for 4 hours priorto measuring the available oxygen of the sample at 500° C. using thegravimetric apparatus. The OSC was 396 μmole O₂/g sample and theNormalized Scattering Intensity I(Q) at Q=0.1 Å⁻¹ was 79. The oxygenrelease rate was 7.6 mg-O₂/m²-min.

EXAMPLE 13

[0089] A mixed nitrate solution was made by combining 88.9 g of anaqueous cerium (III) nitrate solution (28.3 wt % oxide) with 84.9 g ofan aqueous zirconium hydroxy nitrate solution having a Zr:NO₃ ratio ofapproximately 1:1 (26.1 wt % oxide). To this solution was added 5.64 gof yttrium carbonate and 3 g of nitric acid. The mixed solution had asolids content of 26 wt % on an oxide basis. The final oxide compositionwas 50.6 wt % CeO₂ 44.4 wt % ZrO₂ and 5 wt % Y₂O₃. The solution wasadded to 700 ml of 5N ammonia at 40° C. The precipitated hydroxide wastreated with 103 ml of a 3 wt % aqueous hydrogen peroxide solution.

[0090] The molar ratio of hydrogen peroxide to ceria was 0.25. Thefilter cake was re-slurried in water and 15 ppm of Pd, as the nitrate,was added. The resulting mixture was spray dried and calcined at 500° C.for 1 hour. The calcined powder was aged at 1000° C. for 4 hours priorto measuring the available oxygen of the sample at 500° C. using thegravimetric apparatus. The OSC was 344 μmole O₂/g sample and theNormalized Scattering Intensity I(Q) at Q=0.1 Å⁻¹ was 81. The oxygenrelease rate was 4.2 mg-O₂/m²-min.

EXAMPLE 14

[0091] A mixed nitrate solution was made by combining 87.4 g of anaqueous cerium (III) nitrate solution (29 wt % oxide) with 84.1 g of anaqueous zirconium hydroxy nitrate solution having a Zr:NO₃ ratio ofapproximately 1:1 (26.4 wt % oxide). To this solution was added 2.5 g ofgadolinium oxide and 3 g of nitric acid. The mixed solution had a solidscontent of 27 wt % on an oxide basis. The final oxide composition was50.6 wt % CeO₂ 44.4 wt % ZrO₂ and 5 wt % Gd₂O₃. The solution was addedto 700 ml of 5N ammonia at 40° C. The precipitated hydroxide was treatedwith 103 ml of a 3 wt % aqueous hydrogen peroxide solution.

[0092] The molar ratio of hydrogen peroxide to ceria was 0.25. Thefilter cake was re-slurried in water and 15 ppm of Pd, as the nitrate,was added. The resulting mixture was spray dried and calcined at 500 C.for 1 hour. The calcined powder was aged at 1000° C. for 4 hours priorto measuring the available oxygen of the sample at 500° C. using thegravimetric apparatus. The OSC was 384 μmole O₂/g sample and theNormalized Scattering Intensity I(Q) at Q=0.1 Å⁻¹ was 65. The oxygenrelease rate was 4.0 mg-O₂/m²-min.

EXAMPLE 15

[0093] A mixed nitrate solution was made by combining 87.4 g of anaqueous cerium (III) nitrate solution (29 wt % oxide) with 84.9 g of anaqueous zirconium hydroxy nitrate solution having a Zr:NO₃ ratio ofapproximately 1:1 (26.1% oxide). To this solution was added 4 g ofsamarium carbonate (63.4 wt % oxide) and 4 g of nitric acid. The mixedsolution had a solids content of 27 wt % on an oxide basis. The finaloxide composition was 50.6 wt % CeO₂ 44.4 wt % ZrO₂ and 5 wt % Sm₂O₃.The solution was added to 700 ml of 5N ammonia at 40° C. Theprecipitated hydroxide was treated with 103 ml of a 3 wt % aqueoushydrogen peroxide solution.

[0094] The molar ratio of hydrogen peroxide to ceria was 0.25. Thefilter cake was re-slurried in water and 15 ppm of Pd, as the nitrate,was added. The resulting mixture was spray dried and calcined at 500° C.for 1 hour. The calcined powder was aged at 1000° C. for 4 hours priorto measuring the available oxygen of the sample at 500° C. using thegravimetric apparatus. The OSC was 385 μmole O₂/g sample and theNormalized Scattering Intensity I(Q) at Q=0.1 Å⁻¹ was 70. The oxygenrelease rate was 3.8 mg-O₂/m²-min.

EXAMPLE 16

[0095] A mixed nitrate solution was made by combining 87.4 g of anaqueous cerium (III) nitrate solution (29 wt % oxide) with 84.1 g of anaqueous zirconium hydroxy nitrate solution having a Zr:NO₃ ratio ofapproximately 1:1 (26.4 wt % oxide). To this solution was added 10.5 gof calcium carbonate and 3 g of nitric acid. The mixed solution had asolids content of 27 wt % on an oxide basis. The final oxide compositionwas 50.6 wt % CeO₂ 44.4 wt % ZrO₂ and 5 wt % CaO. The solution was addedto 700 ml of 5N ammonia at 40° C. The precipitated hydroxide was treatedwith 103 ml of a 3 wt % aqueous hydrogen peroxide solution.

[0096] The molar ratio of hydrogen peroxide to ceria was 0.25. Thefilter cake was re-slurried in water and 15 ppm of Pd, as the nitrate,was added. The resulting mixture was spray dried and calcined at 500° C.for 1 hour. The calcined powder was aged at 1000° C. for 4 hours priorto measuring the available oxygen of the sample at 500° C. using thegravimetric apparatus. The OSC was 359 μmole O₂/g sample and theNormalized Scattering Intensity I(Q) at Q=0.1 Å⁻¹ was 65. The oxygenrelease rate was 8.5 mg-O₂/m²-min.

EXAMPLE 17

[0097] Example 3 was repeated except, after precipitating the slurry at60° C., the temperature was raised and the slurry was heated at 90° C.for two hours in the mother liquor. The precipitate was then filtered,washed and treated identically as described in Example 3. The structurehad been changed significantly by heating the slurry and the desireddomain structure was destroyed by the high temperature. This resulted ina loss of OSC at 500° C. to a value of only 290 μmole O₂/g sample. Thematerial exhibited a Normalized Scattering Intensity I(Q) of 107 atQ=0.1 Å⁻¹. The oxygen release rate of the material was 4.3 mg-O₂/m²-min.

COMPARATIVE EXAMPLE 1

[0098] 1147 g of water and 136 g of acetic acid were mixed and 236 g ofcerium carbonate was added to form a clear solution of cerium acetate.The mixture was stirred for 48 hours to completely dissolve thecarbonate. To the cerium acetate was added 512 g of zirconium acetate(20 wt % ZrO2) and stirred to make a homogeneous solution. The solutionwas spray dried at 110° C. to form a white powder of mixed acetates. Thepowder was calcined in a muffle furnace to a temperature of 500° C. for1 hour to form the finished mixed oxide.

[0099] The freshly calcined oxide was impregnated with 15 ppm Pd from anaqueous nitrate solution and calcined at 500° C. for 1 hour. The samplewas aged at 1000° C. for 4 hours. The OSC as measured in the TGA at 500°C. was 290 μmole O₂/g sample. The sample was analyzed using SAXS. TheNormalized Scattering Intensity I(Q) was only 40 at Q=0.1 Å⁻¹ as shownin FIG. 2. The Normalized Scattering Intensity I(Q) versus Q was plottedin FIG. 1. The surface area was 1 m²/g. The oxygen release rate was 0.5mg-O₂/m²-min. The final oxide composition was 60 wt % CeO₂, 38 wt % ZrO₂and 2 wt % La₂O₃.

COMPARATIVE EXAMPLE 2

[0100] A mixed oxide powder was prepared using the procedure ofComparative Example 1 except the freshly calcined sample was impregnatedwith 1000 ppm Ni from a nitrate solution. The powder was then calcinedat 500° C. for 1 hour. The sample was aged at 1000° C. for 4 hours. TheOSC as measured in the TGA at 500° C. was 218 μmole O₂/g sample and aNormalized Scattering Intensity I(Q) of 46 at Q=0.1 Å⁻¹. The oxygenrelease rate was 0.2 mg-O₂/m²-min.

COMPARATIVE EXAMPLE 3

[0101] An aqueous cerium (III) nitrate solution was prepared bydissolving 58.6 g of cerium (III) carbonate (49.5 wt % oxide) in 135 gwater and 50.4 g concentrated nitric acid. To this was added 110.5 gzirconyl nitrate (20 wt % oxide). The final mixed nitrate solution had aconcentration of 15.7 wt % solids. 50 g of 30% an aqueous hydrogenperoxide was added to the nitrate solution. Into a heal of 350 g of 70°C. DI water, the peroxide treated nitrate solution was co-precipitatedin a vessel against 300 ml 5N ammonia. A temperature of 70° C. and tomaintain a pH of 8.6. After all of the solution has been added theprecipitate was aged at 70° C. for 0.5 hours.

[0102] The precipitate was filtered, washed with 3 liters of 70° C.water. The washed filter cake was reslurried with water and spray dried.The dried powder was impregnated with 15 ppm Pd as a nitrate solutionand calcined at 500° C. for 1 hour to yield a product having surfacearea >100 m²/g. After aging at 1000 for 4 hours the surface area was 17m²/g. The OSC as measured by TGA under 500° C. isothermal conditions wasonly 274 μmole O₂/g. The sample was analyzed using SAXS. The NormalizedScattering Intensity I(Q) as measured by SAXS was 152 at Q=0.1 Å⁻¹ asshown in FIG. 2. The oxygen release rate was 2.4 mg-O₂/m²-min.

COMPARATIVE EXAMPLE 4

[0103] The filter cake from Comparative Example 4 was insteadimpregnated with 100 ppm Pd prior to calcination. The OSC as measured byTGA under 500° C. isothermal conditions was only 275 μmole O₂/g sample.The Normalized Scattering Intensity I(Q) as measured by SAXS was 120 atQ=0.1 Å⁻¹. The oxygen release rate was 4.0 mg-O₂/m²-in.

[0104] While the present invention has been described in terms ofvarious preferred embodiments, the skilled artisan will appreciate thatvarious modifications, substitutions, omissions, and changes may be madewithout departing from the spirit thereof.

What is claimed is:
 1. A mixed oxide of cerium oxide and zirconium oxidehaving a polyphasic cubic crystalline habit, and an oxygen storagecapacity of greater than 300 μmole O₂/g sample after aging at 1000° C.for 4 hours.
 2. The mixed oxide of claim 1 wherein the oxygen storagecapacity is greater than 315 μmole O₂/g sample after aging at 1000° C.for 4 hours.
 3. The mixed oxide of claim 2 wherein the oxygen storagecapacity is greater than 330 μmole O₂/g sample after aging at 1000° C.for 4 hours.
 4. The mixed oxide of claim 1 wherein the mixed oxide hasan oxygen release rate of greater than 1.0 mg-O₂/m²-min.
 5. The mixedoxide of claim 4 wherein the mixed oxide has an oxygen release rate ofgreater than 2.0 mg-O₂/m²-min.
 6. The mixed oxide of claim 5 wherein themixed oxide has an oxygen release rate of greater than 5.0 mg-O₂/m²-min.7. The mixed oxide of claim 1 wherein the mixed oxide, as determined bya small angle X-ray scattering (SAXS) technique, has a NormalizedScattering Intensity I(Q) of about 47 to about 119 when the scatteringvector Q is 0.10 Å⁻¹.
 8. The mixed oxide of claim 7 wherein the mixedoxide, as determined by a small angle X-ray scattering (SAXS) technique,has a Normalized Scattering Intensity I(Q) of about 50 to 100 whenscattering vector Q is 0.10 Å⁻¹.
 9. The mixed oxide of claim 7 whereinthe mixed oxide, as determined by a small angle X-ray scattering (SAXS)technique, has a Normalized Scattering Intensity I(Q) of about 54 toabout 85 when scattering vector Q is 0.10 Å⁻¹.
 10. The mixed oxide ofclaim 4 wherein the mixed oxide, as determined by a small angle X-rayscattering (SAXS) technique, has a Normalized Scattering Intensity I(Q)of about 47 to about 119 when the scattering vector Q is 0.10 Å⁻¹. 11.The mixed oxide of claim 10 wherein the mixed oxide, as determined by asmall angle X-ray scattering (SAXS) technique, has a NormalizedScattering Intensity I(Q) of about 50 to 100 when scattering vector Q is0.10 Å⁻¹.
 12. The mixed oxide of claim 11 wherein the mixed oxide, asdetermined by a small angle X-ray scattering (SAXS) technique, has aNormalized Scattering Intensity I(Q) of about 54 to about 85 whenscattering vector Q is 0.10 Å⁻¹.
 13. The mixed oxide of claim 1 whereinthe mixed oxide comprises polycrystalline particles of cerium oxide andzirconium oxide.
 14. The mixed oxide of claim 13 wherein the particleshave an average particle size of about 0.1 to about 50 μm.
 15. The mixedoxide of claim 14 wherein the particles have an average particle size ofabout 0.5 to about 20 μm.
 16. The mixed oxide of claim 13 wherein theparticles comprise crystallites having an average crystallite size offrom about 40 to about 200 Å after calcination at 900° C. for 4 hours,as determined using X-ray diffraction.
 17. The mixed oxide of claim 16wherein the crystallites have an average crystallite size of from about50 to about 120 Å after calcination at 900° C. for 4 hours as determinedusing X-ray diffraction.
 18. The mixed oxide of claim 16 wherein thecrystallites comprise a plurality of adjacent domains wherein saidadjacent domains have different cerium to zirconium ratios.
 19. Themixed oxide of claim 18 wherein the domains have an average size of fromabout 10 to about 50 Å.
 20. The mixed oxide of claim 19 wherein thedomains have an average size of from about 10 to about 30 Å.
 21. Themixed oxide of claim 13 wherein the mixed oxide particles have aspecific surface area of at least 30 m²/g after calcination at 500° C.for 2 hours.
 22. The mixed oxide of claim 21 wherein the mixed oxideparticles have a specific surface area of at least 40 m²/g aftercalcination at 500° C. for 2 hours.
 23. The mixed oxide of claim 22wherein the mixed oxide particles have a specific surface area of atleast 50 m²/g after calcination at 500° C. for 2 hours.
 24. The mixedoxide of claim 13 wherein the mixed oxide particles have a specificsurface area of no more than 10 m²/g after calcination at 1000° C. for 4hours.
 25. The mixed oxide of claim 24 wherein the mixed oxide particleshave a specific surface area of no more than 5 m²/g after calcination at1 000° C. for 4 hours.
 26. The mixed oxide of claim 25 wherein the mixedoxide particles have a specific surface area of no more than 3 m²/gafter calcination at 1000° C. for 4 hours.
 27. The mixed oxide of claim1 wherein the mixed oxide comprises about 80 to 20 weight % CeO₂ andabout 20 to 80 weight % ZrO₂.
 28. The mixed oxide of claim 27 whereinthe mixed oxide comprises about 40 to 60 weight % CeO₂ and about 60 to40 weight % ZrO₂.
 29. The mixed oxide of claim 28 wherein the mixedoxide comprises about 50 weight % CeO₂ and about 50 weight % ZrO₂. 30.The mixed oxide of claim 27 wherein the mixed oxide further comprises upto about 10 weight % of a metal oxide other than cerium oxide.
 31. Themixed oxide of claim 30 wherein the metal oxide other than cerium oxideis selected from the group consisting of a rare earth metal oxide otherthan cerium oxide, calcium oxide, and mixtures thereof.
 32. The mixedoxide of claim 31 wherein the rare earth metal oxide is an oxideselected from the group consisting of lanthanum, praseodymium,neodymium, samarium, gadolinium and yttrium.
 33. A process of preparinga polycrystalline mixed oxide particles of cerium oxide and zirconiumoxide, the process comprising: i) providing a mixed salt solutioncomprising at least one cerium salt and at least one zirconium salt in aconcentration sufficient to provide polycrystalline particles of thecorresponding dried mixed oxide product, said particles having a ceriumoxide component and a zirconium oxide component wherein such componentsare distributed within the subcrystalline structure of the particlessuch that each crystallite in the particle is comprised of a pluralityof adjacent domains wherein the Ce:Zr atomic ratios possessed by saidadjacent domains are characterized by a degree of inhomogeneity inrelation to each other, when measured by small angle X-ray scattering,and expressed as being possessed of a Normalized Scattering IntensityI(Q), of from about 47 to about 119 when scattering vector Q is 0.10Å⁻¹, said mixed oxide composition being characterized as having anoxygen storage capacity of at least 260 μmoles O₂/g sample after agingat 1000° C. for 4 hours; ii) treating a mixed salt solution provided inaccordance with step (i) with a base to form a precipitate; iii)treating a precipitate provided in accordance with step (ii) with anoxidizing agent in an amount sufficient to oxidize Ce⁺³ to Ce⁺⁴; iv)washing and drying a precipitate provided in accordance with step (iii);and v) calcining a dried precipitate provided in accordance with step(iv) to obtain polycrystalline cerium and zirconium oxide particles. 34.The process of claim 33 wherein the precipitate is treated with anaqueous dilute hydrogen peroxide to oxidize Ce⁺³ to Ce⁺⁴.
 35. Theprocess of claim 34 wherein the precipitate is treated with dilutehydrogen peroxide in an amount sufficient to provide a molar ratio ofhydrogen peroxide to a Ce of from about 0.25 to about
 1. 36. The processof claim 35 wherein the precipitate is treated with dilute hydrogenperoxide in an amount sufficient to provide a molar ratio of hydrogenperoxide to Ce of from about 0.5 to about
 1. 37. The process of claim 33wherein the mixed salt solution has a solids concentration of at least23 weight % solids, on an oxide basis.
 38. The process of claim 37wherein the mixed salt solution has a solids concentration of greaterthan 25 weight % solids, on an oxide basis.
 39. The process of claim 33wherein the mixed salt solution has a solids concentration of from about24 to about 39 weight % solids, on an oxide basis.
 40. The process ofclaim 39 wherein the mixed salt solution has a solids concentration offrom about 25 to about 29 weight % solids, on an oxide basis.
 41. Theprocess of claim 33 wherein the pH during step ii) is from about 8 to11.
 42. The process of claim 33 wherein the temperature during steps ii)and iii) is not greater than about 80° C.
 43. The process of claim 42wherein the temperature during steps ii) and iii) is not greater thanabout 70° C.
 44. The process of claim 33 wherein the dried precipitateis calcined at a temperature of about 500° C. to about 600° C. for up toabout 6 hours.
 45. The process of claim 33 wherein a dopant is addedfollowing precipitation in step (ii).
 46. The process of claim 45wherein the dopant is added before or after calcination.
 47. The processof claim 46 wherein the dopant is a group VIII transition metal.
 48. Theprocess of claim 47 wherein the dopant is a transition metal selectedfrom the group consisting of nickel, palladium, platinum and mixturesthereof.
 49. The process of claim 33 wherein the precipitate in step iv)is dried by slurrying the precipitate with water and subjecting theaqueous slurry to spray drying.
 50. The process of claim 33 wherein themixed salt solution is prepared by mixing a cerium salt with a zirconiumsalt solution having a cation to anion molar ratio of 1:1 to 1:2. 51.The process of claim 33 wherein the mixed salt solution is prepared bydissolving cerium carbonate into a 1:2 molar ratio zirconium saltsolution and adding a minimum amount of an acid sufficient to dissolvethe carbonate.
 52. A mixed oxide produced by the process of claim 33.53. A catalyst/catalyst support comprising the mixed oxide compositionof claim 1 coated onto a substrate.
 54. A catalyst/catalyst supportcomprising the mixed oxide composition of the mixed oxide composition ofclaim 4 coated onto a substrate.
 55. A catalyst/catalyst supportcomprising the mixed oxide composition of the mixed oxide composition ofclaim 7 coated onto a substrate.
 56. A catalyst/catalyst supportcomprising the mixed oxide composition of the mixed oxide composition ofclaim 10 coated onto a substrate.
 57. The catalyst/catalyst support ofclaim 54 having a noble metal catalyst deposited on the mixed oxidecomposition.
 58. The catalyst/catalyst support of claim 55 having anoble metal catalyst deposited on the mixed oxide composition.
 59. Thecatalyst/catalyst support of claim 56 having a noble metal catalystdeposited on the mixed oxide composition.
 60. A mixed oxide compositionhaving a polyphasic cubic crystalline habit comprising polycrystallineparticles having a cerium oxide component and a zirconium oxidecomponent wherein such components are distributed within thesubcrystalline structure of the particles such that each crystallite inthe particle is comprised of a plurality of adjacent domains wherein theCe:Zr atomic ratios possessed by said adjacent domains are characterizedby a degree of inhomogeneity in relation to each other, when measured bysmall angle X-ray scattering, and expressed as being possessed of aNormalized Scattering Intensity I(Q), of from about 47 to about 119 whenscattering vector Q is 0.10 Å⁻¹.
 61. The mixed oxide composition ofclaim 60 wherein the mixed oxide composition has an oxygen release rateof greater than 1.0 mg-O₂/m²-min.
 62. The mixed oxide composition ofclaim 60 wherein the mixed oxide composition has an oxygen storagecapacity of at least 260 μmole O₂/g sample after aging at 1000° C. for 4hours.
 63. The mixed oxide composition of claim 61 wherein the mixedoxide composition has an oxygen storage capacity of at least 260 μmoleO₂/g sample after aging at 1000° C. for 4 hours.
 64. The mixed oxidecomposition of claim 60 wherein the Normalized Scattering Intensity I(Q)ranges from about 50 to 100 when scattering vector Q is 0.10 Å⁻¹. 65.The mixed oxide composition of claim 64 wherein the NormalizedScattering Intensity I(Q) ranges from about 54 to 85 when scatteringvector Q is 0.10 Å⁻¹.
 66. The mixed oxide composition of claim 60wherein a logarithm plot of the Normalized Scattering Intensity,ln(I(Q)), as a function of the logarithm of the scattering vector,ln(Q), at −2.5<ln(Q)<−1, has a straight line portion and the slope ofthe straight line portion is −4.0±0.4.
 67. The mixed oxide compositionof claim 7 wherein a logarithm plot of the Normalized ScatteringIntensity, ln(I(Q)), as a function of the logarithm of the scatteringvector, ln(Q), at −2.5<ln(Q) <−1, has a straight line portion and theslope of the straight line portion is −4.0±0.4.